ostree/trivial-httpd: Add --random-408s command line option
ostree/trivial-httpd: Add --random-408s command line option

This is exactly like the --random-500s option, except that it will cause
error 408 (request timeout) to be returned, rather than error 500
(internal server error).

This will be used in a following commit to test pull behaviour when
timeouts occur.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #1594
Approved by: jlebon

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree